1. Field of the Invention
This invention relates to tensioning of seabed-to-vessel marine risers. More particularly, this invention relates to tensioning the marine risers with a plurality of pneumatic or hydraulic cylinders.
2. Brief Description of the Related Art
A problem presented by offshore hydrocarbon drilling and producing operations conducted from a floating platform is the need to establish a sealed fluid pathway between each borehole or well at the ocean floor and the work deck of the platform at the ocean surface. A riser typically provides this sealed fluid pathway. In drilling operations, the drill string extends through a drilling riser, the drilling riser serving to protect the drill string and to provide a return pathway outside the drill string for drilling fluids. In producing operations, a production riser is used to provide a pathway for the transmission of oil and gas to the work deck.
The riser is projected up through an opening referred to as a moon pool in the vessel to working equipment and connections proximate an operational floor on the vessel. A riser pipe operating on the floating vessel in water depths greater than about 200 feet (34.72 meters) can buckle under the influence of its own weight and the weight of drilling fluid contained within the riser if it is not partially or completely supported. For floating platforms, a special piece of equipment known as a “riser tensioner” is required to maintain each riser within a range of safe operating tensions as the work deck moves relative to the upper portion of the riser. If a portion of the riser is permitted to go into compression, it could be damaged by buckling or by bending and colliding with adjacent risers. It is also necessary to ensure that the riser is not over-tensioned when the vessel hull moves to an extreme lateral position under extreme wave conditions or when ocean currents exert a significant side loading on the riser.
A tension leg platform (“TLP”) is a type of marine structure having a buoyant hull secured to a foundation on the ocean floor by a set of tethers. The tethers are each attached to the buoyant hull so that the hull is maintained at a significantly greater draft than it would assume if free floating. The resultant buoyant force of the hull exerts an upward loading on the tethers, maintaining them in tension. The tensioned tethers limit vertical motion of the hull, thus substantially restraining it from pitch, roll and heave motions induced by waves, currents and wind. However, unlike conventional platforms which are rigidly attached to the subsea floor, TLPs are not designed to resist horizontal forces induced by waves.
The marine risers have been tensioned in various manners, including the use of counterweights and pneumatic spring systems. The counterweight was the first technique utilized to apply tension to the top of the marine riser. The weight was hung from a wire rope which was run up through wire rope sheaves and down to an upper portion of the riser pipe. The tension was equal to the counterweight and therefore was practicable only for shallow water that required low tension.
The pneumatic spring systems replaced the counterweight systems as deeper water drilling evolved. The pneumatic tensioning devices stored energy in the form of compressed air to apply tension to the top of the riser through wire ropes. The pneumatic tensioning devices typically involved the use of cylinders from which a piston rod was extended, the piston rod having a sheave engaged by the wire rope to be tensioned. The fluid within the hydraulic cylinder was thereby compressed into an accumulator. The cylinder and the accumulator were normally supported by support structures on the floating vessel.
Many tensioner systems in use today act as oil-damped pneumatic springs. A large gas supply keeps a nearly constant pressure above the oil in a gas-oil accumulator. The oil then provides pressure to the face of the piston. As the vessel heaves, the piston moves up and down against a relatively constant force and the tension lines maintain a relatively constant pull.
Many riser tensioners today utilize hydraulically actuated cylinders with pneumatic pressure accumulators to provide the force necessary to maintain the upper portion of the riser within a preselected range of operating tensions. One implementation is accomplished by the use of sheaves attached to the buoyant drilling structure whereby tensioning cables are run over the sheaves and attached to the riser so that the riser is supported by one end of the tensioning cables. The other end of each tensioning cable is connected to a piston of a hydraulic cylinder. The hydraulic cylinders are connected to a relatively large accumulator which maintains the load applied by the cylinders at a relatively constant level over the full range of travel of the pistons. Thus, as the platform moves vertically, the pistons stroke to maintain a relatively constant upward loading on the riser.
Another type of riser tensioner typically used on TLPs also uses a pneumatically pressurized fluid accumulator but eliminates the cables and sheaves used in earlier riser tensioners. Gas and oil accumulators are connected to the cylinders to control the stroke of pistons. The piston rods are directly attached to a riser tensioning ring which supports the riser.
Both classes of riser tensioning systems described generally require separate and relatively large accumulators to maintain the load applied by the cylinders with an acceptable range. Accordingly, it can be appreciated that there still exists a need for an improved riser tensioner system which provides high-capacity tension and provides for limiting peak loads while incorporating high nominal stiffness, and that does not require an excessively large accumulator.